A new study by the Institute for Fiscal Studies (IFS) indicates that children raised by cohabiting couples do no worse on average than children raised by married couples once socio-economic background is taken into account, and therefore there is no good reason on the part of the State to encourage marriage.

To put it another way, it is because the kind of people who get married tend to be better educated that the children of married parents usually do well because the education of the parents rubs off on the children.

But the study completely overlooks the fact that cohabitation is far less stable than marriage and therefore is a much surer path into single parenthood. Decades of rigorous social science studies show that children raised by single parents on average, do not fare as well as children raised by two parents. Even Barack Obama, who was raised by a single mother (plus his grandparents) acknowledges this.

Figures from the British Milennium Cohort Survey (BMCS) show that cohabiting parents are two and a half times as likely to have broken up by the time their children are five compared with married parents.

Furthermore, even marriages that end in divorce tend to last longer than the average cohabiting relationship. The average length in Britain of a marriage that ends in divorce is 11.5 years compared with just two years for a live-in relationship.

The scanty Irish data on the subject suggests that only a quarter of cohabiting couples are still cohabiting after seven years. The result have either broken up or married.

What the data highlighted by the IFS really shows is the extent to which the culture of marriage has collapsed in many disadvantaged areas, the very areas that need the stability of marriage more than any other.